This application relates to and incorporates herein by reference Japanese Patent Application No. 2002-180010 filed on Jun. 20, 2002.
1. Field of the Invention
The present invention relates to a torque sensor with a magnet and a magnetic sensor. For example, the torque sensor detects a torque, which is applied to a transmitting mechanism for transmitting a rotation torque in an electric power steering system of an automotive vehicle.
2. Background of the Invention
A magnetic sensor with a magnet according to a related art is proposed, for example, in JP-A-06-281513. In this sensor, a magnet and a magnetic sensor are fixed to two ends of a torsion bar, respectively. When a torque is applied to the torsion bar, the torsion bar is twisted and a relative angular position between the magnet and the magnetic sensor changes, so that the magnetic sensor detects the position change that is in proportion to the torque. However, the magnetic sensor is fixed to the torsion bar, so that an electrical connection is necessitated for supplying an electric power to the magnetic sensor and for receiving a detection signal from the magnetic sensor. For example, the electrical connection is performed by a slip-ring and a brush. The connection between the slip-ring and the brush has a low reliability.
Another magnetic sensor is also proposed in JP-A-08-159867. In this case, a distortion of the torsion bar is converted to a motion in an axial direction by a gear system. The magnetic sensor can be fixed to a housing, so that no electrical connection is needed. However, the gear system becomes complicated. Moreover, the gear may back-lash or be abraded, so that an error or a response delay may be occurred.
The present invention has an object to simplify a detection mechanism of a torque sensor without any electrical connection.
In the present invention, a torque sensor includes first and second shafts, an elastic shaft, a magnet, first and second ring plates, and a magnetic sensor. The elastic shaft connects the first and second shafts coaxially, and is twistable in accordance with a distortion torque applied to the elastic shaft. The magnet is made of hard magnetic material, is magnetized in an axial direction of the elastic shaft so that the magnet provides a magnetic field around the magnet, and is fixed to the first shaft. The first and second ring plates are made of soft magnetic material, are disposed in the magnetic field provided by the magnet, are fixed to the second shaft, and face each other in the axial direction so that the first and second ring plates sandwich the magnet. The magnetic sensor is disposed in a gap between the first and second ring plates so that the magnetic sensor detects a magnetic flux density in the gap.
Each of the first and second ring plates includes a convexity and a concavity in an inner circumference of each ring plate, respectively. The convexity and the concavity are alternately disposed in a circumferential direction of each ring plate. The convexity of the first ring plate faces the convexity of the second ring plate in the axial direction. The concavity of the first ring plate also faces the concavity of the second ring plate in the axial direction. The first and second ring plates are rotatable against the magnet in accordance with a twist of the elastic shaft, so that the magnetic flux flows in the gap. Thus, the magnetic sensor detects the magnetic flux density in the gap in accordance with a rotation of the first and second ring plates.
In this torque sensor, the magnetic sensor does not contact the ring plates or the magnet. Therefore, the torque sensor has a high reliability.
Preferably, the total number of convexities and concavities in each ring plate are equal to the number of pairs of N pole and S pole in the magnet. In this case, the convexity and the concavity in each ring plate can be positioned easily between the N pole and the S pole of the magnet.
Preferably, the convexity and the concavity are disposed on the entire inner circumference of each ring plate at regular angular intervals so that the convexity and the concavity have the same width in the circumferential direction. The N pole and the S pole are disposed on the entire circumference of the magnet at regular angular intervals so that the N pole and the S pole have the same width in the circumferential direction. Thus, the width of the convexity or the concavity is substantially equal to the width of the N pole or the S pole. In this case, a positioning error between the convexity or the concavity and the N pole or the S pole is averaged and substantially cancelled.
Preferably, when the elastic shaft is not twisted, i.e., when the elastic shaft is at a neutral position, a center of each convexity and concavity in the circumferential direction is substantially coincident with a boundary between the N pole and the S pole of the magnet in the circumferential direction. In this case, the neutral position of the elastic shaft is not shifted even when a magnetic force of the magnet changes in accordance with an ambient temperature change. So an accuracy of the torque sensor around the neutral position is secured easily.
Preferably, the torque sensor includes a spacer, which is made of nonmagnetic material, is disposed between the first and second ring plates, and is disposed outside of the magnet. The spacer prevents an iron powder from adhering the magnet; so that the magnetic force of the magnet does not decrease.
Preferably, the torque sensor includes a plurality of concentration rings. Each concentration ring is disposed on the first and second ring plates, respectively, and is made of soft magnetic material. Each concentration ring concentrates a magnetic flux flowing from the magnet through each ring plate, respectively. The magnetic sensor is disposed between the concentration rings so that the magnetic sensor detects a magnetic flux density concentrated by the concentration rings. In this case, a total magnetic flux flowing from the entire ring plate is averaged, and the magnetic flux density is concentrated. A detection error caused by a dispersion of manufacturing parts or by a dispersion of assembling parts of the torque sensor, a centering deviation between the first and second shafts are suppressed by the concentration of the magnetic flux.